A.3.6—Mechanical energy

Mechanical energy stores

Mechanical energy is the sum of translational kinetic energy, gravitational potential energy and elastic potential energy:

$E_{mech}=E_k+E_{p,g}+E_{p,elastic}$

Use the chosen system

Mechanical energy describes these stores within the system. Internal energy, chemical energy and sound may also be present in the full energy account but are not mechanical energy.

Common trap

Do not call all conserved energy mechanical energy; classify the store before applying a mechanical-energy equation.

How it is assessed

The evidence asks for a speed from gravitational potential energy and tests the relation between kinetic energy and total energy at terminal velocity.

Command terms

Show / Identify

What earns marks

Identify which energy stores are mechanical and apply the relevant relation. For a falling object, distinguish kinetic-energy increase from gravitational potential-energy decrease and note that terminal motion may transfer energy to internal stores.

Watch for

Calling thermal or chemical energy mechanical energy, or assuming total energy equals kinetic energy during terminal motion.

Representative question

[Maximum number: 1]

show that the speed of the ball is about $4.3 \mathrm{~ms}^{-1}$ .

$V=\sqrt{2 \times 9.81 \times 0.95}$ OR =4.32 《m ${ }^{-1}$ 》

Must see either full substitution or answer to at least 3 s.f.

A.3.6—Mechanical energy

Identify Mechanical Energy

A.3.6 Exam Analysis

Assessment in practice

Question 1